Among chairs used in offices and the like, there are chairs in which a backrest is tiltably attached to a support structure. Further, as a chair of this type, a chair in which a support structure which is a supporting member and a backrest which is a supported member are connected via a reaction force mechanism capable of adjusting a reaction force is known (for example, refer to Patent Document 1).
The reaction force mechanism disclosed in Patent Document 1 has a structure in which a plurality of unit biasing parts are provided in a pivotally connecting portion between a supporting member (support structure) and a supported member (backrest) in an axial direction of a pivot shaft and a combination of the unit biasing parts which causes a reaction force to be effective between the supporting member and the supported member can be selected by an operation lever. The reaction force mechanism is a mechanism which adjusts the reaction force acting between the supporting member and the supported member by switching the effective combination of the unit biasing parts. Therefore, as compared with a mechanism which adjusts the reaction force by changing an initial load of a single biasing part, it is possible to reduce an operation force required to adjust the reaction force.